No problem Tom - i'm just passing on knowledge i learned from someone else :)

As for the sponge tool/"saturation with the process", there is an interesting side effect of using the dodge and burn tools - it alters the colours as well as the luminosity so if you wanted to just affect the luminosity(monochromatic contrast), just change the blend mode on the layer you are dodge/burning to "Luminosity", (always dodge and burn on a duplicate layer) - flick back to "Normal" blend mode to see the change in colours.  Personally i like the effect it has on things when dealing with some subject types (landscape colours seem to pop with the effect).

As for the Sponge in particular - i havent really used it much... sometimes when there is a colour cast on a piece of white but i like the colour cast on the rest of the image i would use it rather than mask a saturation layer, but apart from that i dont use the Sponge tool. For largescale selective desaturation it is easier and quicker to use a saturation layer and masking.

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography